According to a research, which was published in American Journal of Obstetrics & Gynecology, sperm was found to be in greater numbers, with faster swimming speeds and fewer abnormalities in semen made during the winter, with a steady decline in quality from spring onward.
The researchers found that men with normal sperm production had the healthiest sperm in the winter.
It was also mentioned that previous research has found that sperm counts around the world are declining. It could be a more sedentary lifestyle to chemicals in the environment that has effect on sperm health. [source]